This invention relates to a main steam line apparatus for use with a nuclear reactor plant, especially a boiling water nuclear reactor.
In the case of a boiling water reactor, steam produced in a pressure vessel typically is introduced into a turbine through a main steam line apparatus. The main steam line apparatus generally includes piping through which the steam flows, a main steam valve, a main steam control valve and main steam isolation valves, all of which are located in the piping.
When an abnormal condition occurs in the turbine or a generator, the main steam valve is closed rapidly and the steam supply to the turbine is stopped. The rapid closure of the main steam valve can generate a pressure wave in the piping which travels back toward the pressure vessel. The steam normally produced by the pressure vessel cannot be supplied due to the adverse pressure gradient in the piping caused by the wave, and accordingly, the pressure in the pressure vessel can experience a sudden increase.
An increase in the pressure in the pressure vessel will cause a corresponding increase in the pressure in the coolant channels of the core. Consequently, voids (steam bubbles) normally present in the core coolant channels are collapsed and the thermal neutron flux in the core increases momentarily because of the increase in the ability of the coolant to moderate the fast neutron flux to thermal levels. The increase in the thermal neutron flux produces a corresponding increase in the nuclear reaction rate, causing the temperature of the nuclear fuel elements to become unacceptably high. The temperature of the coolant-moderator in the core channels increases dramatically as a consequence, leading to a further increase in pressure vessel pressure.